Related-art image forming apparatuses such as copiers, facsimile machines, and printers include a scanner that serves as an image reading device that employs a scanning unit or an image reading unit to read image of a document. In the scanning unit, while a moving member that is driven by a stepping motor is moving along a reading object surface of the document placed on a contact glass, a light source of the moving member emits a light beam via the contact glass onto the reading object surface of the document. Then, the scanner unit directs the light beam reflected by the reading object surface of the document to an image sensor to read the image formed on the reading object surface of the document in a sub-scanning direction, which is a direction of movement of the moving member of the scanner unit.
As the moving member moves, it starts reading the image of the document using as a reference a home position that is a stop position. Therefore, the image reading device includes a position sensor such as a photo interrupter to detect the position of the moving member. When the moving member is detected by the home position sensor, the moving member is stopped by the stepping motor at the home position, which is a predetermined distance from the home position sensor, to stand by.
Then, when a scan of the document is required by a central processing unit (CPU), the stepping motor moves the moving member to a reading start position in the sub-scanning direction using as a reference the home position and starts scanning the document (as discloses, for example, in Japanese Patent Application Publications Nos. 2009-206623 and 2006-010718.)
Generally, a home position sensor is attached to the housing of the image reading device, with the document positioned at one end of the contact glass, which is mounted on the housing. This one end of the contact glass where the document is positioned in the sub-scanning direction is the reading start position of the document. Conventionally, a distance from the home position to the reading start position is managed by pulse signals output by the stepping motor, and thus it is necessary that the home position sensor detect the document accurately.
However, with the home position sensor attached to the housing and the positioning of the document set to the one end of the contact glass in the sub-scanning direction thereof, in a case in which the pulse signals output by the stepping motor manage the distance from the home position to the reading start position, if the housing undergoes thermal expansion or contraction, the relative positions of the reading start position and the home position in the sub-scanning direction also change.
In this case, the reading start position with respect to the home position based on the position of the moving member as detected by the home position sensor deviates from the correct reading start position. Consequently, the reading start position of the image is deviated by the amount of deviation of the reading start position, and as a result, the image reading accuracy deteriorates.